module Firehose
  module Server
    # Connects to a specific channel on Redis and listens for messages to notify subscribers.
    class Channel
      attr_reader :channel_key, :redis, :subscriber, :list_key, :sequence_key

      def self.redis
        @redis ||= EM::Hiredis.connect
      end

      def self.subscriber
        @subscriber ||= Server::Subscriber.new(EM::Hiredis.connect)
      end

      def initialize(channel_key, redis=self.class.redis, subscriber=self.class.subscriber)
        @channel_key, @redis, @subscriber = channel_key, redis, subscriber
        @list_key, @sequence_key = Server.key(channel_key, :list), Server.key(channel_key, :sequence)
      end

      def next_message(last_sequence=nil, options={})
        last_sequence = last_sequence.to_i

        deferrable = EM::DefaultDeferrable.new
        # TODO - Think this through a little harder... maybe some tests ol buddy!
        deferrable.errback {|e| EM.next_tick { raise e } unless [:timeout, :disconnect].include?(e) }

        # TODO: Use HSET so we don't have to pull 100 messages back every time.
        redis.multi
          redis.get(sequence_key).
            errback {|e| deferrable.fail e }
          redis.lrange(list_key, 0, Server::Publisher::MAX_MESSAGES).
            errback {|e| deferrable.fail e }
        redis.exec.callback do |(sequence, message_list)|
          Firehose.logger.debug "exec returned: `#{sequence}` and `#{message_list.inspect}`"
          sequence = sequence.to_i

          if sequence.nil? || (diff = sequence - last_sequence) <= 0
            Firehose.logger.debug "No message available yet, subscribing. sequence: `#{sequence}`"
            # Either this resource has never been seen before or we are all caught up.
            # Subscribe and hope something gets published to this end-point.
            subscribe(deferrable, options[:timeout])
          elsif last_sequence > 0 && diff < Server::Publisher::MAX_MESSAGES
            # The client is kinda-sorta running behind, but has a chance to catch
            # up. Catch them up FTW.
            # But we won't "catch them up" if last_sequence was zero/nil because
            # that implies the client is connecting for the 1st time.
            message = message_list[diff-1]
            Firehose.logger.debug "Sending old message `#{message}` and sequence `#{sequence}` to client directly. Client is `#{diff}` behind, at `#{last_sequence}`."
            deferrable.succeed message, last_sequence + 1
          else
            # The client is hopelessly behind and underwater. Just reset
            # their whole world with the lastest message.
            message = message_list[0]
            Firehose.logger.debug "Sending latest message `#{message}` and sequence `#{sequence}` to client directly."
            deferrable.succeed message, sequence
          end
        end.errback {|e| deferrable.fail e }

        deferrable
      end

      def unsubscribe(deferrable)
        subscriber.unsubscribe channel_key, deferrable
      end

      private
      def subscribe(deferrable, timeout=nil)
        subscriber.subscribe(channel_key, deferrable)
        if timeout
          timer = EventMachine::Timer.new(timeout) do
            deferrable.fail :timeout
            unsubscribe deferrable
          end
          # Cancel the timer if when the deferrable succeeds
          deferrable.callback { timer.cancel }
        end
      end
    end
  end
end